1. Field of the Invention
The present invention relates to Magnetic Tunnel Junction (MTJ) Magnetoresistive (TMR) devices for use as magnetic field sensors, such as read heads for reading magnetically recorded data, as memory cells in nonvolatile magnetic random access memory (MRAM) cells, and for magnetic logic and spintronic applications. More particularly, the present invention relates to magnetic tunneling elements and MTJ devices having significantly improved magnetoresistance.
2. Description of the Related Art
Magnetic Tunnel Junctions (MTJ) are promising candidates for memory storage cells by enabling a nonvolatile, high performance Magnetic Random Access Memory (MRAM). An MTJ-based MRAM has the potential to rival conventional Dynamic Random Access Memory (DRAM) in density and cost, and conventional Static Random Access Memory (SRAM) in speed. In addition, MRAM is truly nonvolatile, that is, the state of the memory is maintained even after the power has been removed from the memory. Furthermore, each MTJ bit can be read non-destructively without changing its magnetic state. For MTJ-MRAM to directly replace conventional semiconductor memory technologies, however, it is preferred that the materials making up an MTJ memory are built on complementary metal oxide semiconductor (CMOS) circuits that are necessary to read and write the state of the MTJ cells. For example, MTJ-MRAM arrays having a cross-point architecture are described in U.S. Pat. No. 6,097,625 to Scheuerlein and U.S. Pat. No. 5,640,343 to Gallagher et al.
An MTJ includes two ferromagnetic layers separated by a thin insulating layer, wherein the resistance (or conductance) through the layers depends on the relative orientation of the magnetic moments of the ferromagnetic layers. The change in resistance as the orientation of the moments is changed from parallel to anti-parallel divided by the resistance for the parallel state is the tunneling magnetoresistance (TMR). The most useful MTJ for memory cells has the magnetic moment of one of the ferromagnetic (FM) layers (termed a storage layer) free to rotate and the magnetic moment of the other ferromagnetic layer fixed or pinned (termed a reference layer) by being exchange-biased with a thin antiferromagnetic (AF) layer.
In a cross-point MRAM, the MTJ devices are switched by the application of two magnetic fields that are applied along two orthogonal directions, the “easy” and “hard” axes of the MTJ device. Typically, the magnetic easy and hard anisotropy axes are defined by the shape of the MTJ element through the self demagnetizing fields. The MRAM array contains a series of MTJ elements arranged along a series of word and bit lines, typically arranged orthogonal to one another. For example, FIG. 1 depicts an exemplary cross-point array 100 of an MTJ-MRAM. Cross-point array 100 includes a plurality of MTJ memory cells 101, a plurality of row lines 102 (also referred to as word lines), and a plurality of column lines 103 (also referred to as bit lines). An MTJ memory cell 101 is located at an intersection of a row line 102 and a column line 103. The magnetic switching fields are realized by passing currents along the word and bit lines. All of the cells along a particular word or bit line are subjected to the same word or bit line field. Thus, the width of the distribution of switching fields for the selected MTJs (those subject to the vector sum of the bit and word line fields) must be sufficiently narrow that it does not overlap the distribution of switching fields for the half-selected devices. The magnetic moment within the MTJ devices lies along a particular direction, which is referred to as the “easy” axis. The orthogonal direction is the magnetic “hard” axis.
One of the most challenging problems for the successful development of MTJ memory storage cells is to obtain sufficiently uniform switching fields for a large array of MTJ cells. This uniformity can be characterized in various ways. One method is to use a parameter termed the “array quality factor” (AQF). The AQF represents the mean switching field at zero hard-axis field divided by the standard deviation of the same distribution for an array of MTJ devices. The AQF parameter is useful when the observed distribution of switching fields follows a Gaussian distribution. In some cases, though, the observed distribution of switching fields may not follow a simple Gaussian form, for example, when the magnetic elements may have two different ground states having similar energies.
Based on a statistical model that takes into account the MTJ memory cell size, shape, and pitch along the word and bit lines together with the pattern of written states of the MTJ elements for an 1 Mbit array, it is estimated that an AQF above ˜10 to 20 is needed for reliable write operation (write yield close to 100%) of such a MRAM chip having elliptical cells formed at 0.18 micron ground rules. The AQF required is sensitive to details of the writing procedure. The observed AQF is influenced by various factors including the lithographic patterning of the individual MTJ storage elements, as well as details of the process integration, especially through the dielectric material surrounding the patterned edges of the MTJ elements, and by the MTJ materials and structure itself. One clear limitation on the AQF is the polycrystalline nature of the MTJ materials, which becomes especially important for high density MRAM in which the size of the MTJ element is so small that the MJT element contains only a small number of crystalline grains. It has been hypothesized that for very small MTJ cells, the relatively small number of crystalline grains having random anisotropy axes can lead to significant variations in magnetic switching field from device to device. Improved AQFs may be obtained by either reducing the crystalline grain size or by using amorphous ferromagnetic (a-FM) materials having no macroscopic granular structure.
MTJs with a-FM storage layers, in addition to having a narrower distribution of magnetic switching fields, are also expected to have other improved magnetic properties, such as lower values of the coercive fields Hc. The latter property is advantageous as this allows for reduced write currents. Typically, an amorphous FM will have a lower magnetic moment than its crystalline counterpart because the alloy is made amorphous by the addition of non-magnetic elements, thereby diluting the magnetization. This aspect can be advantageous for MRAM applications because the self-demagnetization fields, which are directly proportional to the magnetization of the ferromagnet for otherwise identical structures, would thereby be reduced. The Curie temperature (Tc) of amorphous alloys can be varied by varying the alloy composition, but can be sufficiently high for MRAM and other applications. For example, this may be useful for thermally-assisted writing techniques. See, for example, U.S. Pat. No. 6,538,919 to Abraham et al. Similarly, other magnetic properties, for example, the magnetostriction of the amorphous alloy can often be tuned to the required value by choice of the alloy composition. In this regard, it is useful to be able to add a considerable number of different constituents to the amorphous alloy to be able to engineer the magnetic properties of the alloy as is desired.
Amorphous FM films are also expected to have improved elastic properties because such films will resist plastic deformation. The lack of an ordered atomic lattice implies the absence of dislocations. More importantly, a-FM layers should exhibit improved corrosion resistance due to the absence of grain boundaries along which contaminants can diffuse. Similarly, the thermal stability of MTJs having a-FM layers will be more thermally stable because diffusion of material is enhanced along grain boundaries in thin film materials.
Previous studies have disclosed that magnetic storage layers consisting of CoFeB or CoFeNbB, or CoNbB a-FM alloys yield MTJs or giant magnetoresistive (GMR) devices having softer magnetic characteristics (i.e., lower coercivity) than equivalent MTJs or GMR devices having crystalline magnetic layers. See, for example, U.S. Pat. No. 6,436,526 to Odagawa et al. Also, see U.S. Pat. No. 6,028,786 to Nishimura, which describes the use of CoFeB alloys in an MTJ stack.
What is needed is an MTJ element having a high TMR and a thermal stability that can withstand typical MRAM processing thermal budgets.